1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a facsimile machine and a printer, for forming an image on a recording material to obtain a hard copy based on an electrophotographic process.
2. Description of the Related Art
In many conventional image forming apparatuses utilizing the electrophotographic process, a corona charger has been employed as means for electrically charging a drum type electrophotographic photoconductor (hereinafter referred to as a xe2x80x9cphotoconductorxe2x80x9d) that serves as an image carrier. The corona charger is arranged in a non-contact and opposed relation to the photoconductor and the photoconductor surface is exposed to discharge corona generated by the corona charger so that the photoconductor surface is electrically charged to a predetermined potential with a predetermined polarity.
On the other hand, a contact charger (direct charger) has recently been put into practical use because of superior advantages over the corona charger, i.e., less ozone and lower power consumption. With a contact charger, a charging member, to which a voltage is applied, is contacted with a photoconductor so that the photoconductor surface is electrically charged to a predetermined potential with a predetermined polarity. A contact charger using a magnetic brush, as the charging member, is employed in many cases because of advantages such as a good charging ability and safety in contact. In a magnetic brush type contact charger, conductive magnetic particles are magnetically retained on a magnet directly or on a sleeve incorporating a magnet to serve as a magnetic brush. The magnetic brush is contacted with the photoconductor surface while the photoconductor is stopped or rotated. By applying a voltage to the magnetic brush in such a condition, charging of the photoconductor is started. Alternatively, a brush made up of conductive fibers (fur brush) or a conductive rubber roll fabricated by forming conductive rubber into a roll shape can also be used as the contact charging member.
As another type of contact charging, an injection charging method is also known in which a charge injection layer is provided in a photoconductor and a charging member, to which a voltage is applied, is contacted with the photoconductor to inject charges into the charge injection layer so that the photoconductor surface is electrically charged to a predetermined potential with a predetermined polarity. With this injection charging method, the photoconductor can be charged to have a surface potential substantially identical to an applied DC voltage (DC bias) regardless of whether or not an AC voltage (AC bias) is applied to the charging member in a superimposed manner. Thus, since the photoconductor is electrically charged without utilizing a discharge phenomenon that occurs in the case of employing the corona charger, the charging can be realized with generation of no ozone and lower power consumption.
Furthermore, in recent years, a so-called cleaner-less system has also been put into practical use for the purposes of reducing the apparatus size, simplifying the construction, and not producing waste toner from the viewpoint of environmental friendliness. In the cleaner-less system, a cleaning device for removing toner from the photoconductor surface remaining after transfer of a toner image onto a recording (transfer) material, e.g., a sheet of paper, is omitted. After recovering the toner remaining after the transfer by a contact charging device, the toner is ejected from the contact charging device to be recovered by a developing device during a period in which an image is not formed.
By employing the cleaner-less system and the injection charging method, a smaller and simpler image forming apparatus generating no ozone, consuming lower power and recovering the leftover toner can be obtained.
FIG. 12 is a schematic view of a laser beam printer as a conventional image forming apparatus. The laser beam printer comprises a photoconductor 1 serving as an image carrier, a magnetic brush 3 serving as a contact charging means, an exposure device 100, a developing device 4, and a transfer device 7 serving as transfer means. The components 3, 100, 4, and 7 are successively disposed around the photoconductor 1 in the rotating direction (denoted by arrow A) thereof.
In an image forming mode, the photoconductor 1 is driven by a driving means (not shown) to rotate in the direction of arrow A. During the rotation, the photoconductor surface is uniformly electrically charged (with a negative polarity) by the magnetic brush 3 serving as a contact charging means. Then, the uniformly charged surface of the photoconductor 1 is subjected to exposure of an image by the exposure device (laser scanning device) 100 using a laser beam, whereby an electrostatic latent image corresponding to image information is formed on the photoconductor 1. The electrostatic latent image is developed into a toner image through a reversal process by the developing device 4.
When the toner image on the photoconductor 1 reaches a transfer nip 70 between the photoconductor surface and a transfer belt 71 of the transfer device 7, a recording material P in a cassette 41 is supplied by a sheet supply roller 42 and then fed to the transfer nip 70 by a register roller 43 in a timed relation. Then, charges having a polarity opposite to that of the toner are applied to the backside of the recording material P from a transfer charging blade 74, to which a transfer bias is applied, whereby the toner image on the photoconductor 1 is transferred onto the front side of the recording material P. The recording material P having the transferred toner image is separated from the surface of the transfer belt 71 with the aid of a separation charger 15, and then fed to a fusing device 6. The toner image is fused into a permanently fixed image on the surface of the recording material P by the fusing device 6, and thereafter the recording material P is ejected from the image forming apparatus.
On the photoconductor 1 having passed the transfer nip 70, there exists, though in a small amount, toner that has not been transferred onto the recording material P at the transfer nip 70 (i.e., after-transfer remaining toner). The after-transfer remaining toner is electrostatically and physically scraped off by the magnetic brush 3 and is temporarily absorbed by the magnetic brush 3. As the after-transfer remaining toner accumulates inside the magnetic brush 3, the resistance of the magnetic brush 3 itself is increased to such an extent that the magnetic brush 3 can no longer sufficiently charge the photoconductor 1. This produces a potential difference between the magnetic brush 3 and the surface of the photoconductor 1, whereupon the after-transfer remaining toner so far retained by the magnetic brush 3 is caused to electrostatically move onto the photoconductor 1. The after-transfer remaining toner having moved onto the photoconductor 1 is electrostatically taken in by the developing device 4 and then consumed in a next cycle of image formation.
On the other hand, toner remaining on the surface of the transfer belt 71, from which the recording material P has been peeled off, is removed by a transfer belt cleaner 92 constituted by a urethane rubber blade to be ready for a next cycle of image formation.
FIG. 13 is a schematic view of a color laser beam printer as a conventional 4-drum full-color image forming apparatus. In this color laser beam printer, rotary drum type photoconductors 1a to 1d serving as image carriers are provided in respective image forming stations. Magnetic brushes 3a to 3d serving as contact charging means, exposure devices 100a to 100d, developing devices 4a to 4d, and transfer devices 7 (transfer charging blades 74a to 74d) are disposed respectively around the photoconductors 1a to 1d. 
In an image forming mode, the photoconductors 1a to 1d are driven to rotate about respective central support shafts at a predetermined circumferential speed (process speed). During the rotation, the photoconductor surfaces are uniformly electrically charged with a negative polarity by the magnetic brushes 3a to 3d serving as contact charging means.
Then, the uniformly charged surfaces of the photoconductors 1a to 1d are subjected to scan exposure of laser beams modulated corresponding to image signals of respective colors (yellow, magenta, cyan and black) output from the exposure devices (laser scanning devices) 100a to 100d, whereby electrostatic latent images corresponding to image information of the respective colors are successively formed on the photoconductors 1a to 1d. The electrostatic latent images formed on the photoconductors 1a to 1d are developed by the respective developing devices 4a to 4d. More specifically, a yellow toner image is developed by the developing device 4a, a magenta toner image is developed by the developing device 4b, a cyan toner image is developed by the developing device 4c, and a black toner image is developed by the developing device 4d in succession.
On the other hand, recording materials P, e.g., sheets of paper, stocked in a sheet supply cassette 41 are supplied one by one by a sheet supply roller 42 and then fed by a register roller 43 at predetermined timing to a transfer nip between the photoconductor 1a and the transfer device 7, serving as transfer means. Then, the toner image on each photoconductor 1a to 1d is transferred onto the recording material P in succession.
Finally, the recording material P having the transferred toner images is separated from the surface of a transfer belt 71 with the aid of a separation charger 15, and then passes a fusing device 6 in which the toner is fused and fixed under heat and pressure. Thereafter, the recording material P having a permanently fixed image is ejected from the image forming apparatus.
Detection and control of toner density will be described below.
Toner density is conventionally detected, for example, by an optical or magnetic detection method utilizing the fact that the light reflectance or the magnetic permeability of a developer, i.e., a mixture of toner and carriers, is changed depending on the toner density. However, the optical detection method has the problem that a transparent window for viewing the developer is stained with the toner itself. The magnetic detection method has the problem that the bulk density of the developer is changed depending on the temperature and the humidity, and therefore errors are caused in the magnetic permeability. Further, for the purpose of feedback in control of the finally required image density, a deterioration in charging power of the photoconductor 1a to 1d and the charging means 3a to 3d must also be taken into consideration. Thus, it is desired to measure the image density after transfer, which is closer to the final image density based on the toner density and the charging power, and to feed back the measured result to the density control.
In view of the above, one conventional method of detecting the toner density comprises the steps of forming an image-density measuring test pattern on the photoconductor 1a to 1d at a position outside the area of an image transferred onto the recording material P, transferring the test pattern onto the transfer belt 71 at a position outside the image area to obtain an image density closer to the final image density, and detecting the intensity of light reflected from a toner image of the test pattern.
Further, in addition to the density control, for registering position shifts of images among a plurality of image carriers, there has also been employed a method of forming a position-shift detecting test pattern on the transfer belt 71, reading the test pattern to detect the position shift, and feeding back the detected result in position shift control.
Such a test pattern in the form of a toner image, which has been formed on the transfer belt 71 and from which the image density has been read, is likewise removed by the transfer belt cleaner 92.
In the above-described image forming apparatuses each utilizing the cleaner-less system, the after-transfer remaining toner on the photoconductor 1a to 1d is reused and hence a great improvement of toner utilization factor is expected. However, since the test pattern in the form of a toner image is formed for feedback control of, e.g., the toner density for stabilizing the density of an output image, leftover toner is generated, though in a small amount. Also, for treating the leftover toner, the transfer belt cleaner 92 and a recovery container for the leftover toner recovered from the transfer belt cleaner 92 are required.
Moreover, when the transfer belt cleaner 92 and the recovery container are located far away from each other from limitations in layout of the internal structure of an apparatus body, a leftover toner transport passage, and the like are required. Additionally, the generation of leftover toner is disadvantageous in that users are required to exchange the recovery container and the amount of consumed toner is increased.
With the view of solving the problems set forth above, it is an object of the present invention to provide an image forming apparatus which can improve the toner utilization factor.
To achieve the above object, an image forming apparatus according to one aspect of the present invention includes an electrostatic latent image forming unit for forming an electrostatic latent image on a surface of an image carrier, a developing unit for developing the electrostatic latent image with toner, a transferring unit for transferring a toner image on the image carrier onto a transfer medium in a transfer area, a test pattern forming unit for forming a test pattern, which is made of toner and used for image control, on the transfer medium, and a control unit for detecting the test pattern and executing image control. The transferring unit transfers the test pattern on the transfer medium, which has been subjected to detection, onto the image carrier, and the developing unit recovers the test pattern having been transferred onto the image carrier.
Also, an image forming apparatus according to another aspect of the present invention includes a plurality of electrostatic latent image forming units for forming electrostatic latent images on surfaces of a plurality of image carriers, a plurality of developing units for developing the electrostatic latent images on the plurality of image carriers with toners of different colors, a plurality of transferring units for transferring toner images on the plurality of image carriers onto a transfer medium in respective transfer areas, test pattern forming units for forming test patterns, which are made of toners of different colors and used for image control, on the transfer medium, and a control unit for detecting the test patterns and executing image control. The plurality of transferring units transfer the test patterns on the transfer medium, which have been subjected to detection, onto the image carriers corresponding to respective colors of the toners forming the test patterns, and the developing units associated with the image carriers, onto which the test patterns have been transferred, recover the corresponding test patterns.
Further objects, features and advantages of the present invention will become apparent from the following description of the preferred embodiments with reference to the attached drawings.